Why is texture paint showing a different UV map
This came down to a problem between UV unwrapping and my modifier stack. All modifiers need to be applied before unwrapping, otherwise there will be glitches in the UV.
In the question above, I had a sub-d and bevel modifier being applied dynamically, and this lead to inconsistencies in the UV map. By applying both and unwrapping a second time my UV islands began to appear in texture paint.